Chemotherapy as cancer therapy has found clinical application in the treatment of almost every type of cancer. One of the major problems in cancer chemotherapy is the development of resistance to cytotoxic drugs. Patients who do not respond to a first course of chemotherapy relapse because tumor cells develop resistance against chemotherapeutic agents or has acquired resistance to cytotoxic agents used in a previous treatment. A tumor may also manifest resistance to a cytotoxic agent to which it has not been previously exposed. Multidrug resistance (MDR) in tumor cells has a significant impact on the efficacy of cancer chemotherapy and appears as a major obstacle in the modern cancer treatment. MDR is mainly related to the expression of the adenosine triphosphate ATP-binding cassette (ABC) transporters. P-glycoprotein (P-gp) (the best studied target for reverting MDR), multidrug resistance-associated protein (MRP1) and the breast cancer resistance protein (BCRP) as a major MDR proteins actively transport a wide variety of structurally different substrates out of the tumor cells, thereby decreasing their intracellular concentrations. Many actual chemotherapeutic agents are considered as potential P-gp, MRP1 and BCRP substrates (Szakács et al, Nat Rev Drug Discov 3: 219-34 (2006); Szakács et al., Drug Resistance in Cancer Cells 1-20 (2009)).
From all numerous efforts to overcome MDR like transcription control of P-gp expression the most promising approach has been the development of MDR modulators that are able to increase the intracellular drug levels in co-application with MDR substrates by the efflux pump inhibition. Substances of different groups have been used as P-gp inhibitors. Ca2+ channel blocker verapamil is the most investigated and often used as reference compound, but unfortunately, in combination with actual anticancer drugs cardiotoxicity was observed (Pennock et al., J Natl Cancer Inst 83: 105-10 (1991)).
The functional unit of an ABC transporter contains two transmembrane domains (TMDs) and two nucleotide ATP-binding domains (NBDs). Transporters such as ABCG2 (BCRP) contain only one TMD and one NBD forms dimers.
Several ABC transporters have been found to be overexpressed in cancer cell lines under selective conditions. It was shown that the major mechanism of MDR in most cultured cancer cells involves P-gp, MRP1 and BCRP transport proteins. ABC transporters control not only the drug release to the cell, but also the intracellular compartmentalization or division between the cytoplasm and nucleus.
P-gp, a member of the ABCB subfamily, confers the strongest resistance to the wide variety of compounds. P-gp transports vinca alkaloids, anthracyclines, epipodophyllotoxins and taxanes. P-gp is normally expressed in epithelium of the liver, kidney and gastrointestinal tract at pharmacological barrier sites in stem cells and cells of immune system.
MRP1 is a member of ABCC subfamily and confers resistance to several hydrophobic compounds that are also P-gp substrates. However, MRP1 can export glutathione, glucuronate or sulphate conjugates of organic anions. MRP1 is expressed in wide range of tissues, tumors and cancer cell lines.
BCRP is a member of ABCG subfamily. The substrate specificity of BCRP overlaps considerably with that of P-gp. BCRP is involved in the mechanism of resistance to a topoisomerase I inhibitor (topotecan) or topoisomerase II inhibitor (mitoxantrone). BCRP does not act on paclitaxel or vincristine transport, which are excreted by P-gp, and BCRP is involved in excretion of a camptothecin derivative, which is barely transported by P-gp (Kruijtzer et al., J Clin Oncol 20: 2943-50 (2002)). BCRP is expressed in many normal tissues, including liver, placenta, brain, hematopoietic stem cells and other types of stem cells.
Besides the clinically important drugs, several fluorescent compounds are transported by P-gp, MRP1 and BCRP such as rhodamine 123 (P-gp), calcein (MRP1), Hoechst 33342 (P-gp, BCRP). These fluorescent compounds are used in studies of ABC transporters in cell lines.
Therefore, there is an actual need in MDR modulators, which are non-toxic, have weak influence (or no influence) on cardiovascular system and would effectively inhibit adenosine triphosphate binding cassette transport—modulate multidrug resistance in tumor cells and rise effectiveness of chemotherapy.
At the same time, thieno[2,3-b]pyridines are known to be biologically active substances, possessing, for example, PI3K inhibition activity, antiviral activity, osteogenesis promotion activity and modulating properties towards metabotropic glutamate receptors.
Thus, thieno[2,3-b]pyridines having phosphatidylinositide 3-kinases (PI3K) inhibitors activity have been disclosed in WO 2012/003262 A1.
Thieno[2,3-b]pyridines exhibiting nicotinamide adenine dinucleotide phosphate (NADPH) oxidase II inhibitor activity have been disclosed in WO 2011/075559 A1.
Thieno[2,3-b]pyridines proposed as inhibitors of human mitogen-activated protein kinase (MEK) enzymes are disclosed in WO 2009/153554 A1, WO 2009/013462 A1, WO 2009/093008 A1, WO 2007/088345 A1.
Thieno[2,3-b]pyridines possess antiviral activity, and more specifically are useful for treating HIV (Human Immunodeficiency Virus) infection (WO 2010/130842 A1, WO 2009/062288 A1) or Hepatitis C (US 2006/0019976 A1).
Thieno[2,3-b]pyridines have been reported to be useful for promoting osteogenesis, suppressing bone resorption and/or improving bone density; for prevention or treatment of osteopathy (for example, osteoporosis, in particular postmenopausal osteoporosis, senile osteoporosis or secondary osteoporosis caused by the use of steroids or immunosuppressants), osteopenia or bone destruction associated with rheumatoid arthritis, Paget's disease of bone, bone fracture or dysostosis due to dwarfism or osteoarthritis (EP 1764367 A1).
Thieno[2,3-b]pyridines have been claimed as potent modulators of metabotropic glutamate receptors (for mGluR5 and mGluR1 receptor subtype reported in WO 2007/072090 A1 and WO 2007/072091 A1, mGluR2 receptor subtype reported in WO 2006/030031 A1).
Thieno[2,3-b]pyridines have IκB kinase (IKK) complex inhibitor activity, therefore are useful in the treatment of IKK mediated diseases including autoimmune diseases, inflammatory diseases, cardiovascular disease and cancer have been disclosed in US 2007/0293533 A1.
Thieno[2,3-b]pyridines as inhibitors of tumor necrosis factor (TNF) have been disclosed in WO 2006/074919 A2.
Thieno[2,3-b]pyridines have been reported as vanilloid receptor 1 (VR1 or TRPV1) antagonists in WO 2006/068618 A1.
Thieno[2,3-b]pyridines are active on the GABAB receptor and can be used in treating CNS disorders (WO 2006/063732 A1).
Thieno[2,3-b]pyridines which modulate K+ channel (possess inhibitor activity) have been described in WO 2006/061642 A1.
Thieno[2,3-b]pyridines that are selective allosteric modulators of the M4 subtype of muscarinic receptors, useful for treatment of disorders associated with M4 muscarinic receptors have been reported in WO 2006/047124 A1.
However, there are no reports of thieno[2,3-b]pyridines exhibiting multidrug resistance modulating properties.